2 answers

Literally: "By thinking and thinking, Nothing happens... Though I may think a thousand times." It means that you cannot experience the One by thinking or reasoning. The experience of Sat Naam lies beyond the mind.

You can find an in depth analysis of this very first pauri of Japji here. Well worth reading.

Soche means taking bath getting yourself cleaned, Mind does not become pure even if you take bath thousands of times
(souchae:by bathing, soch: cleanliness or purity, lakh vaar: thousands of times). Guru sahib singh we must follow his hukam what Guru Sahib says to you through bani.